This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. All ACERT c.w. spectrometers currently utilize conventional ca 100KHz magnetic field modulation for ESR spectral line detection. However, for high-field detection of moderately broad spectral linewidths the utility of this detection technique is severely constrained. Detection of moderately broad HF spectra corresponding to, e.g., spin-labeled proteins in the intermediate slow-motion regime, becomes relatively inefficient when [unreadable]Bmod <<[unreadable]Hpp, necessitating operation of field modulation components at the extreme limit of power input capability, corresponding to a maximum field modulation of only 5 - 10Gpp. At field values exceeding this modulation level, coil-induced Lorentz forces become an increasingly significant microphonic noise source, and the coil power dissipation rapidly becomes a significantly perturbing heat source. Ideally, modulation widths of ca 100G would be available for a majority of HF samples, in particular biological samples at low concentrations,;however, this value is clearly out of reach of conventional direct magnetic field modulation techniques. An alternative modulation solution we have previously investigated is the circular-dichroism scheme described in the 2006 ACERT grant renewal proposal, Core Project III, at (F). For spectra that are not too broad, however, source frequency modulation (FM), a simpler and more easily implemented method of broad ESR line detection, seems possible (Hyde et al., 2007). Our initial task will be to evaluate the performance of a wide-band FM line detection system of at least 280 MHzpp (100Gpp) maximum modulation capability which may be adapted to our existing 170/240 GHz c.w. spectrometer. Because this HF spectrometer utilizes a bolometer power detector rather than a typical hetrodyne conversion and detection scheme as described elsewhere in the literature, our FM line detection system will differ in key design aspects with respect to FM systems described elsewhere. We believe, for example, that it is likely FM detection SNR in a bolometer (or other baseband power detection) system can be greatly enhanced by subtracting out the frequency- (i.e., field-) dependent baseline prior to synchronous demodulation. We have assigned this project a very high priority since improved HF c.w. ESR sensitivity is a critical factor in reading biological samples of low concentration.